The present invention relates to a method for manufacturing a bottle-shaped can, of which a can trunk, a shoulder portion and a neck portion having a threaded portion are integrally formed, of a metallic sheet having a thickness of 0.1 to 0.4 mm, and a tool for use in the method.
More particularly, the invention relates to a forming method for forming especially the shoulder portion into a smooth and beautiful slope not in a step shape or a shape having a step mark left, when the bottle-shaped can is to be manufactured by working the bottom side of the can formed into a bottomed cylindrical shape, to form the shoulder portion having an inclined annular face and a diametrically small cylindrical neck portion integrally, and a tool for use in the method.
As beverage cans for various soft drinks or beer, there are generally employed the DI cans (Drawn and Ironed cans), of which the can trunk (or side wall portion) and the can bottom are integrally formed by drawing and ironing a metallic sheet such as an aluminum alloy sheet or a surface-treated steel sheet.
Specifically, this DI can has its body formed by integrally forming the bottom portion having a shape of a high pressure resistance and a trunk portion thinned by the drawing and ironing workings and by necking in the open upper end of the trunk portion to reduce a diameter of the open upper end. The can body is filled with a drink such as a soft drink or beer, and the diametrically reduced open upper end is seamed with an easy open end (i.e., an end sheet having an easy opening) having a smaller diameter than that of the trunk. These cans are shipped as canned drinks.
As disclosed in WO 81/01259, on the other hand, there is also practiced the bottomed cylindrical can which is formed to have a thinner trunk wall than a bottom wall by drawing and re-drawing (or bending and extending at the re-drawing time) the surface-treated steel sheet laminated on its two sides with a thermoplastic resin film. The can thus manufactured is necked in like the DI can so that it may be used as the beverage can.
As the containers for various soft drinks, on the other hand, there have been employed in recent years the bi-oriented molded container made of a polyethylene terephthalate resin (i.e., the PET bottle). Accordingly, there have been manufactured various beverages contained in the PET bottles which can be repeatedly sealed with threaded caps.
These beverage PET bottles have an advantage over the above-described can containers for beverages in that the PET bottles can be repeatedly sealed with the caps. However, the PET bottles are in considerably lower states than those of the can containers in the recycling ratio for collecting and recycling the resources. Therefore, it has been investigated to enhance the conveniences of the can containers by adding re-sealing function to the can containers having the high resource recycling ratio.
In the prior art, there are disclosed in Japanese Patent Laid-Open No. 10-509095 (WO96/15865) several types of bottle-type DI cans having shapes similar to those of the PET bottles, i.e., the DI cans which have threaded neck portions to be screwed and closed with the threaded caps.
These DI cans are classified into: the type in which an end sheet to be seamed on the open upper end of a can trunk is formed integrally with a threaded neck portion; the type in which the threaded neck portion is integrally formed by reducing the diameter of the open upper end side of the can trunk stepwise by the neck-in working; and the type in which the diametrically small neck portion and the shoulder portion having a slope are formed by drawing the bottom portion side (or the end wall portion) of a cup at multiple steps, in which the trunk portion of the cup is then ironed into a thin trunk portion and in which a threaded portion is formed in the neck portion whereas the can end is seamed on the open end of the trunk portion. In the above-specified Laid-Open, there are disclosed not only the structures of the bottle-shaped cans of the individual types but also the forming methods.
According to the disclosure of Japanese Patent Laid-Open No. 58-47520, on the other hand, at the time of drawing the can trunk, the bottom portion is drawn into a convex stepped shape, and this convex stepped shape is re-drawn at a subsequent ironing time, to form a convex stepped portion having a diametrically small cylindrical neck portion and a square shoulder portion in the bottom portion (or the end wall portion) of the DI can. This neck portion is threaded and sealed with the threaded cap. After this DI can was filled with beverage from the end opening of the trunk portion, this end opening is sealed by seaming the can end.
In Japanese Patent Laid-Open No. 64-62233, moreover, there is disclosed that the DI can drawn and ironed is pressed (or drawn) at its bottom to form a diametrically small cylindrical neck portion and a frusto-conical shoulder portion (having a shape of a frustum of a cone) and that a thread is then formed in or a threaded cylindrical portion is mounted on the neck portion.
Of the aforementioned bottle-shaped cans which can be sealed again with the threaded cap, the can of the type in which the threaded neck portion is formed integrally with the end sheet is formed at its body into the DI can or a bottomed can such as the DTRD can (Drawn, Thin and Re-Drawn) formed by being drawn and bent/extended (or stretched) or the can formed by being bent/extended (or stretched) and ironed. The can trunk is filled with a content such as beverage, and then the open upper end of the can trunk is seamed and sealed with the end sheet having the threaded neck portion formed integrally therewith. According to the bottle-shaped can of this type, therefore, the can body has a shape substantially identical to that of the existing general can, and enjoys an advantage that few changes are required in the filling facilities to suppress the cost for the facilities.
In the bottle-shaped can of this type, however, the end sheet seaming portion is located in the upper portion of the can to raise problems that dust is liable to accumulate in the concave portion inside of the seamed portion, and that the seamed portion itself protrudes to deteriorate the appearance.
In the bottle-shaped can of the type in which the neck portion is formed not at the end sheet but integrally at the upper end portion of the can body, on the other hand, the upper end portion is stretched thin by a similar working as the can body is drawn and ironed or bent and extended. Considering the later step of working the neck portion, therefore, the upper end portion of the can body may be worked so relatively thick as to make the extension of the material less than the lower portion. Since the neck portion has a considerably smaller diameter than that of the can body, however, the diametrical reduction ratio for forming the neck portion is so large that the neck portion is difficult to constrict all at once by enlarging one drawing rate. In addition, it is desirable to reduce the cap size so as to lower the cost for the material and accordingly to reduce the diameter of the neck portion more than the external diameter of the can body. In order to satisfy these desires, it is necessary to make the diametrical reduction ratio more for forming the neck portion by drawing the open upper end of the can body, and this necessity requires multiple steps of neck-in workings.
For example, the can to be relatively frequently used as the drink can for beer has a trunk diameter of 66 mm (of 211 diameter), and twenty to thirty necking steps are required if the neck portion of such can is to be necked in to a diameter of 25.4 mm. Thus, in the bottle-shaped can having the neck portion formed by constricting the open upper end of the can body, a number of necking machines are required to raise the cost for the facilities, and the increase in the number of working steps makes it frequent to damage or deform the can thereby to lower the quality of the can.
In the bottle-shaped can of the type in which the shoulder portion and the neck portion are formed by working the can bottom, on the contrary, the can bottom portion or the portion to be formed into a portion of the shoulder portion and the neck portion is hardly affected by the working to form the can so that the working is applied to the portion having no work hardening and having a thickness substantially equal to that of the original metallic sheet. When the can bottom is to be drawn, moreover, its diameter can be reduced while being unwrinkled. As compared with the case in which the neck portion is formed by necking in the upper portion of the aforementioned can trunk, therefore, one drawing extent can be increased to reduce the diameter more by one step thereby to reduce the number of steps drastically for forming the neck portion.
Although one drawing extent can be made larger than that of the neck-in working, however, there is a limit to the drawing ratio (i.e., the reduction ratio of one drawing extent). The limit of the drawing ratio in the drawing case with the unwrinkling function is more or less different for the materials and is about 1.5 for a beverage can of a metallic sheet such as a beer can. When a neck portion (having diameter of 25.4 mm) of a diametrically small cylindrical shape is formed by drawing the flat bottom of the bottomed cylindrical DI can (having a diameter of 66 mm), it is necessary to repeat the drawing step three or four times.
As a method for forming the diametrically small neck portion integrally with the bottom side of the DI can, there has been disclosed in Japanese Patent Laid-Open No. 58-47520 a method for manufacturing the DI can having a diameter of 67.83 mm and made of a tinned steel sheet. At a re-drawing step, the bottom portion of the can is re-drawn to form a convex stepped portion having a diameter of 26 mm. This convex stepped portion of the can bottom is re-drawn at the final stroke of the subsequent ironing step, to form the shoulder portion having the so-called xe2x80x9csquare shouldered portionsxe2x80x9d and the cylindrical neck portion having a height of 6 mm and a diameter of 16 mm.
In Japanese Patent Laid-Open No. 64-62233, there is disclosed a method for forming a frusto-conical shoulder portion and a cylindrical neck portion by pressing (or drawing) the bottom of the DI can formed into the bottomed cylindrical shape.
According to the former method, however, the height of the neck portion is too short at 6 mm to form a threaded portion capable of retaining sufficient sealing properties. According to the latter method, although the drawing step of multiple stages is not shown in the Drawings of the Specification, it is apparent from FIGS. 2 and 3 that the can trunk has a thickness of two to three times of that of the bottom end, and it is understood from the description of the Specification anticipating the threading of the neck portion that a relatively thick (e.g., about 0.6 to 1.5 mm) aluminum alloy sheet or stainless steel sheet is employed as the trunk material. When this thick material is employed, less wrinkles are formed by the drawing even at a high drawing ratio. Even if so, many drawing steps have to be repeated for forming the diametrically small cylindrical neck portion and the frusto-conical shoulder portion (or a truncated conical shoulder portion). Although the wrinkling can be suppressed, therefore, step shapes or many circumferential marks according to the number of drawing steps are left on the shoulder portion. These step shapes are identical to a plurality of convex and concave steps or ribs, as shown in FIG. 28 of Japanese Patent Laid-Open No. 10-509095, or a plurality of convex and concave steps or circumferential beads 108 shown in FIG. 32.
Here, the problems of the step shapes at the multiple drawing steps by the pressing or the circular shock marks formed when those step shapes are smoothed are described, as follows, in Japanese Patent Laid-Open No. 55-107638 disclosing a method for manufacturing a cup-shaped end for small-sized beer bottles.
In the prior art, after the multiple drawing steps for drawing the can end into a cup shape, a stretching (or a final forming) is performed to finish the end into a predetermined smooth cup shape and to size the cover. By the multiple pre-drawing steps in these working steps, a plurality of a concentric and annular convex portions are formed on the outer face of the cup-shaped end body. These are the portions which are formed by the outer circumference edges of the leading end of a punch having different diameters. These annular convex portions have to be turned backward of their warped direction so that they may be eliminated. In this case, the surface structure of the material, as once extended, receives an inverse compression stress so that the aforementioned shock marks are formed to lower the commercial value seriously.
These shock marks look like streaks not only to degrade the appearance but also to lower the corrosion resistance. On the other hand, the shock marks also occur on the inner face of the end body to separate the coating and cause the corrosion at the shock marks when the material is exemplified by the aluminum alloy sheet having the coating of an epoxy resin on the inner and outer faces, so that the can is unsuited as the can for confining food. When the aluminum alloy sheet is employed for the food can, therefore, the can has to be surface-treated, after pressed, by oxidizing or coating it so that an increase in cost cannot be avoided.
In the invention disclosed in Japanese Patent Laid-Open No. 55-107638, therefore, it is intended to prevent the shock marks from appearing, by leaving the annular convex and concave portions formed by the multiple drawing steps on the cup-shaped end body.
If these annular convex and concave portions are on the upper face of the container, however, it cannot be said that the appearance is excellent. In the container of this shape, on the other hand, dust is liable to accumulate in the concave portions while the container is displayed in the shop and cannot be easily wiped off, to raise a problem that the appearance is degraded.
By repeating the drawing of the flat bottom of the DI can three or four times, as described before, the diametrically small cylindrical neck portion and the shoulder portion having the slope can be formed integrally with the can body, to provide a bottle-shaped can having a shape resembling the PET bottle having a round transverse section, as employed as the ordinary beverage container. At the individual drawing steps of the forming steps, the unwrinkling has to be performed with individual tools. Therefore, the ring-shaped and the step-shaped portions corresponding to the shape of the inner circumference end edges of the drawing die are so formed at the portion or the shoulder portion of the bottle-shaped can according to the number of drawing (or re-drawing) steps, and a clear boundary line is formed between the cylindrical portion and the slope portion. In order to eliminate those step-shaped portions and the boundary line thereby to provide a smooth slope, it is conceivable to perform the pressing working by using a pair of reforming tools having a curved slope of a domed longitudinal section or a slope of a straight longitudinal section, thereby to reform the shoulder portion which is formed profiling the surface shapes of those forming tools. Even with this pressing working, however, the step-shaped portions and the clear boundary line between the cylindrical portion and the slope portion are left as the forming marks of ring shape to degrade the appearance.
This will be described in more detail. According to the method for manufacturing the diametrically small cylindrical portion and the sloped shoulder portion gradually by repeating a plurality of drawing steps, as described above, the portion, as formed before re-drawn as the circumferential boundary line between the diametrically small cylindrical portion and the slope, is left in an apparent state as the circular mark in a portion of the sloped shoulder portion leading downward to the cylindrical portion which has been re-drawn into a smaller diameter.
If the drawing working is performed four times, three circular step portions (or three step-shaped portions) or boundary line marks are clearly left on the shoulder portion. These circular step portions or marks cannot be eliminated even after the shoulder portion was reformed.
Specifically, the portion, as has been the boundary line between the diametrically small cylindrical portion and the slope, is clearly left as the mark of the circular step portion or the boundary line on the shoulder portion which is newly formed at the subsequent drawing step, and this mark cannot be eliminated in the prior art even by reforming the shoulder portion.
The circular mark thus far described will not seriously affect the function of the container but will make an important point as the commercial goods. Specifically, the image of a commodity is expressed by the appearance of the container so that the beverage maker always demands for a design to stimulate the purchasing wills of the consumers. When the can is to be manufactured, therefore, it is an important point of design to form the shoulder portion from the neck portion to the can trunk into a smooth and beautiful curved slope of a domed longitudinal section or a smooth and beautiful slope of a straight longitudinal section. Hence, the aforementioned forming mark is earnestly desired to disappear because it is a fatal defect in the design.
Here in Japanese Patent Laid-Open No. 10-509095, especially in its FIGS. 18 to 27, there is disclosed a method for forming a cup trunk portion (or a side wall portion) into a diametrically small and thin cylindrical trunk portion. According to this method, the cup, as drawn from a blank punched out from a metallic sheet, is first drawn at its bottom portion repeatedly by several times (preferably, three or more) to form a diametrically small cylindrical neck portion. Next, the neck portion of the bottom portion is bulged at the domed shoulder portion. After this, the cup trunk portion (side wall portion) is re-drawn and ironed to form the diametrically small and thin cylindrical trunk portion.
According to the disclosed method, however, when the can trunk is formed, the metallic sheet material of the neck portion, as formed at the bottom of the cup, is pulled through the shoulder portion into the trunk portion of the can trunk as the metallic sheet material moves from the side wall portion of the cup to the thin trunk portion of the can trunk. As a result, the cylindrical shape of the neck portion, as formed at the bottom portion of the cup, cannot be kept in the initial shape so that the cylindrical vertical wall of the neck portion turns into a frusto-conical tapered wall. In this state, there arises a disadvantage that a predetermined thread cannot be formed at the step of threading the neck portion. In order to keep the sealing performance between the neck portion and the threaded cap, therefore, there arises a problem that the neck portion and the shoulder portion have to be reformed so as to raise the cylindrical neck portion as the vertical wall from the shoulder portion bulged in the domed shape.
A main object of the invention is to provide a bottle-shaped can manufacturing method capable of easily forming the aforementioned neck portion and the smooth shoulder portion leading to the neck portion.
More particularly, an object of the invention is to eliminate the boundary line, which has existed between the diametrically small cylindrical portion and the slope formed at steps from the first step of drawing the diametrically small cylindrical portion to the last but one drawing step, to such an extent as is hardly discriminated after the shoulder portion is reformed, although the elimination has been impossible in the prior art.
Specifically, an object of the invention is to provide a method for manufacturing a bottle-shaped can of a metallic sheet. The can bottom portion is drawn by a plurality of times to form a shoulder portion having a curved slope of an arcuate longitudinal section or a slope of a straight longitudinal section, and a diametrically small cylindrical neck portion. Even if the drawing working is repeated by a plurality of times to reduce the diameter of the neck portion to a predetermined diameter, a shoulder portion having a smooth and beautiful curved slope of domed shape or a slope of a straight longitudinal section is formed without apparently leaving the circular boundary line or its mark between the cylindrical portion and the slope portion, as formed by the drawing workings of several times, on the shoulder portion.
In order to achieve the above-specified objects, according to the invention, there is provided a bottle-shaped can manufacturing method for forming a shoulder portion having a slope and a diametrically small cylindrical neck portion integrally by further working the bottom side of a bottomed cylindrical can which is formed thinner at its trunk wall than at its bottom wall by drawing a metallic sheet having a thickness of 0.1 to 0.4 mm and by executing at least one thinning working of a bending/extending working and an ironing working, comprising: a step of preforming the bottom corner portion of the can into a curved shoulder face having an arcuate longitudinal section (i.e., a curved face to be formed into a portion of the shoulder portion); a first diametrically small cylindrical portion forming step of drawing the bottom of the can, with the curved shoulder face of the bottom corner portion being unwrinkled, into a diametrically smaller bottomed cylindrical shape than a trunk portion by using an unwrinkling pusher having the curved face shape of the shoulder portion on an outer face shape of its leading end portion, a drawing die having the curved face shape of the shoulder portion on an inner face shape of its leading end portion, and a drawing punch; a second diametrically small cylindrical portion forming step of drawing the bottomed cylindrical portion drawn from the can bottom, with the bottom corner portion being unwrinkled, into a diametrically smaller bottomed cylindrical shape, by using: an unwrinkling pusher having a tapered face having a substantially straight longitudinal section profiling a tangent line to an arcuate longitudinal section of a virtual curved face leading to the preformed curved shoulder face, a re-drawing die having a tapered face of a straight longitudinal section profiling a tangent line to an arcuate longitudinal section of a virtual curved face leading to the curved shoulder face at its portion to face at least the tapered face of the pusher, and a re-drawing punch; and a shoulder portion reforming step of pushing and extending the shoulder portion, which is formed by the first diametrically small cylindrical portion forming step and the second diametrically small cylindrical portion forming step, after the diameter of the bottomed cylindrical portion formed by executing the drawing working of the second diametrically small cylindrical portion forming step once or two or more times becomes substantially equal to that of the neck portion, into a smooth slope leading to the curved shoulder face on the trunk side.
According to the bottle-shaped can manufacturing method of the invention, the flat can bottom, as enclosed by the can bottom corner portion having the curved shoulder face formed in advance, of the bottomed cylindrical can is subjected to the first drawing by using the drawing die having the shape of the curved shoulder face on the inner face of its leading end and the unwrinkling pusher having the shape of the curved shoulder face on the outer face of its leading end portion, to preform the preformed curved shoulder face shape without wrinkling below the diametrically small bottomed cylindrical portion.
Even if the drawing step for forming the diametrically small neck portion is repeated a plurality of times, on the other hand, the second diametrically small cylindrical portion forming step or the second or later drawing step is performed by using the unwrinkling pusher having at its leading end portion the tapered face having a substantially straight longitudinal section profiling a tangent line to an arcuate longitudinal section of the virtual curved face leading to the curved shoulder face and the re-drawing die having at its leading end portion the tapered face of a substantially straight longitudinal section profiling a tangent line to an arcuate longitudinal section of a virtual curved face leading to the curved shoulder face. As a result, the shoulder portion is formed in the same surface shape or its repeated shape as the surfaces of the die and the pusher used at each drawing working. At the subsequent reforming steps, therefore, those tapered faces can be pushed and extended and can be reformed to form the shoulder portion having the smooth slope leading to the curved shoulder face of the trunk side.
On the other hand, the second diametrically small cylindrical portion forming step may include: the step of re-drawing the diametrically small cylindrical portion, with the bottom corner portion of the drawn diametrically small cylindrical portion being unwrinkled, by using an unwrinkling pusher having at its leading end portion a tapered face having a substantially straight longitudinal section profiling a tangent line to an arcuate longitudinal section of a virtual curved face leading to the curved shoulder face, a re-drawing die having at its leading end portion such a tapered face having the shape of a straight longitudinal section profiling a tangent line to an arcuate longitudinal section of the virtual curved face leading to the curved shoulder face as has a larger external diameter than that of the tapered face of the pusher, and a re-drawing punch; and the step of continuing the re-drawing working till the boundary line between the diametrically small cylindrical portion and the slope and the slope portion in the vicinity of the boundary line come into contact with the tapered face of the pusher and the tapered face of the die. Moreover, in the shoulder portion reforming step, one or two or more tapered faces of the straight longitudinal section may be pushed and extended into a smooth domed curved face leading to the curved shoulder face on the trunk side.
According to the bottle-shaped can manufacturing method of the invention, therefore, even if the drawing step of forming the diametrically small cylindrical portion is repeated a plurality of times on the flat can bottom which is enclosed by the bottom corner portion of the bottomed cylindrical can having the curved shoulder face formed in advance, at the second diametrically small cylindrical portion forming step or the second and later drawing step, the re-drawing is performed by using the pusher and the die which have at their individual leading end portions the tapered faces of the generally straight longitudinal section profiling a tangent line to an arcuate longitudinal section of the virtual curved face leading to the curved shoulder face and of which the die has a larger external diameter of the tapered face of the substantially straight longitudinal section than that of the pusher, i.e., a lower end portion of the tapered face of the die is positioned on the outside of a lower end portion of the tapered face of the pusher in the radial direction with respect to a can axis, and the drawing working is performed till the individual tapered faces of the die and the pusher come into contact with both the boundary line between the diametrically small bottomed cylindrical portion and the slope, as formed by the preceding drawing step, and the slope portion in the vicinity of the boundary line, so that neither the clear boundary line nor its mark is left on the shoulder portion.
Specifically, the boundary line (or the bent portion) between the slope and the diametrically small cylindrical portion, that is, the portion formed at the preceding step into the shape profiling the corner shape on the inner face of the leading end portion of the drawing die or the redrawing die comes, as the re-drawing proceeds, into abutment against the tapered face of the re-drawing die on the outer face of the can, so that the boundary line is made unclear (or the bent portion is shallowed) when it is pulled toward the diametrically small bottomed cylindrical portion while receiving the frictional resistance from the tapered face. When the boundary line passes through the clearance between the re-drawing die and the unwrinkling pusher being pushed toward each other, the unclear boundary line (or the shallowed bent portion) and the slope in the vicinity of the former are flattened by the pulling force, so that the portion to be formed to lead to the initial curved shoulder face does not become a step portion having the clear boundary line (or the deep bent portion) left.
Moreover, the shoulder portion is formed by the plurality of drawing steps into one or two or more tapered faces of a substantially straight longitudinal section, which are jointed by a low convex portion or ridge. At the shoulder portion reforming step, the tapered faces are pushed and extended into a continuous smooth domed curved face so that the shoulder portion jointing the diametrically larger cylindrical trunk portion and the diametrically smaller neck portion can be formed into a smooth and beautiful curved face of an arcuate longitudinal section leading to the curved shoulder face on the trunk side, with little forming mark (or the mark of the boundary line), as formed by the multiple drawing steps.
In the method of the invention, on the other hand, the tool to be used at the shoulder portion re-drawing step includes a pair of forming tools having a surface shape of a virtual curved face extending from the curved shoulder face, and the shoulder portion is reformed in its entirety into a smooth curved face of a domed longitudinal section leading to the curved shoulder face, by pinching most of the shoulder portion between the paired forming tools to push and extend the same.
By pushing and extending the shoulder portion by employing the paired forming tools having the surface shape of the virtual curved face extending from the curved shoulder face, therefore, the shoulder portion can be formed in its entirety into the smooth curved face of the arcuate longitudinal section leading to the curved shoulder face on the trunk side.
In the method of the invention, on the other hand, the second diametrically small cylindrical portion forming step may include: the step of re-drawing the diametrically small cylindrical portion, with the bottom corner portion of the previously drawn diametrically small cylindrical portion being unwrinkled, by using an unwrinkling pusher having at its leading end portion a slope having a substantially straight longitudinal section profiling a tangent line to an arcuate longitudinal section of a virtual curved face leading to the curved shoulder face, a re-drawing die having, at its portion to face the slope of the pusher, a slope having the shape of a straight longitudinal section profiling a tangent line to an arcuate longitudinal section of the virtual curved face leading to the curved shoulder face, at its portion on the leading end side from the slope, a convex curved face of an arcuate longitudinal section having a larger external diameter than that of the slope of the pusher, and a re-drawing punch; and the step of continuing the drawing working till the boundary line (or the bent portion) between the diametrically small cylindrical portion and the slope and the slope portion in the vicinity of the boundary line come into contact with the surfaces of the pusher and the die.
In the bottle-shaped can manufacturing method of the invention, at the second and later diametrically small cylindrical portion re-drawing step, therefore, both the boundary line (or the bent portion) between the diametrically small cylindrical portion and the slope, as formed at the preceding step into the shape profiling the corner shapes of the inner faces of the leading end portions of the drawing die and the re-drawing die, and the portion of the slope in the vicinity of the boundary line are brought, as the drawing proceeds, into abutment against the convex curved faces of the die so that the boundary line is made unclear (or the bent portion is shallowed) while it is pulled toward the diametrically small bottomed cylindrical portion while receiving the frictional resistance from the convex curved face. Moreover, the boundary line (or the bent portion) and its adjacent slope portion are flattened by the pulling force, when they pass through the clearance between the pusher and the die pushed toward each other, and the slope near the diametrically small cylindrical portion, as newly formed, is changed into a shallow concave curved face by receiving the convex curved face shape of the die transferred thereto.
This shallow concave curved face is easily formed into the smooth slope at the shoulder portion reforming step.
In the method of the invention, moreover, the slopes and convex curved faces of the unwrinkling pusher and the re-drawing die, as used when the second diametrically small cylindrical portion forming step is repeated two or more times, may be individually identical to those of the unwrinkling pusher and the re-drawing die, as used at the first step of the second diametrically small cylindrical portion forming step.
At the plurality of re-drawing steps, therefore, a plurality of shallow concave curved faces are formed in the slope portion for the shoulder portion so that the smooth slope is easily formed at the shoulder portion reforming step.
In the method of the invention, on the other hand, the tool to be used at the shoulder portion re-drawing step may include: a pair of forming tools having a surface shape of the tapered face of the straight longitudinal section profiling a tangent line to a virtual curved face leading from the curved shoulder face; and a punch to be inserted into the diametrically small cylindrical portion. Most of the shoulder portion can be formed into a smooth slope continuing in the straight longitudinal section shape leading to the curved shoulder face, by pinching the shoulder portion in its entirety between the paired forming tools and by applying a pushing force to the bottom portion of the diametrically small cylindrical portion thereby to push and extend the shoulder portion.
Therefore, most of the shoulder portion is pinched by the paired forming tools having the surface shapes of the tapered faces of a straight longitudinal section profiling a tangent line to a virtual curved face leading from the curved shoulder face, and the pushing force is applied to the bottom portion of the diametrically small cylindrical portion by the punch. As a result, the pulling force toward the diametrically small cylindrical portion is applied to the shoulder portion so that the shoulder portion having a plurality of shallow concave curved faces is pushed and extended, and so that the portion pinched by the paired forming tools takes the shape of the tapered face of the same straight longitudinal section as the surface shape of the forming tools.
As a result, at the shoulder portion reforming step, there is formed the shoulder portion which continuously leads to the curved shoulder face on the trunk side and which mostly has a smooth slope of continuous straight longitudinal sections.
In the method of the invention, on the other hand, a curved shoulder face is formed on the bottom corner portion of the can by preforming the bottom corner portion by using a punch having a curved face on the outer circumference of its leading end portion after the bottomed cylindrical can was formed by the drawing working and the thinning working and before the bottom side of the can is drawn.
Since the preparatory forming step is made independent, therefore, not only a curved face having a large radius of curvature but also a curved face having a small radius of curvature can be easily formed on the can bottom corner portion. By preforming the curved shoulder face on the bottom corner portion, on the other hand, it is possible to reform the curved shoulder face without wrinkling the lower end portion of the diametrically small cylindrical portion when forming the diametrically small cylindrical portion at the next drawing working step, and finally to make smooth and continuous either the cylindrical trunk portion and the curved face of a domed section or the cylindrical trunk portion and the shoulder portion having a slope of the straight longitudinal section.
In the method of the invention, on the other hand, the curved shoulder face is performed on the bottom corner portion of the can at the final forming step of the bottomed cylindrical can by using a punch having a curved face at the outer circumference of its leading end portion, as the punch to be used at the final working step of forming the bottomed cylindrical can which is made thinner at its trunk wall than at its bottom wall by the drawing working and by the thinning working.
At the final step of forming the cylindrical can having a thinner trunk wall than the bottom wall, therefore, the can bottom corner portion is curved to combine the trunk wall thinning step and the shoulder portion curved face preforming step into one step so that the number of working steps can be reduced.
In the method of the invention, on the other hand, the metallic sheet can be prepared by laminating a thermoplastic resin film on an aluminum alloy sheet in advance.
Since the aluminum alloy sheet is laminated on its two sides with the thermoplastic resin film, therefore, this thermoplastic resin film plays the role of a lubricant when the bottomed cylindrical can is to be formed by the drawing working or the thinning working, when the can is drawn on its bottom side or when the neck portion is threaded. When the aluminum alloy sheet is extended or bent, the laminating the thermoplastic resin film layer is accordingly extended or bent to reduce the consumption of the lubricant and to make the aluminum alloy sheet hard to be damaged with the working tools.
Since the laminating state with the thermoplastic resin can be kept even after the bottle-shaped can was formed, moreover, no new protecting coating is required later.
In the method of the invention, on the other hand, the metallic sheet is prepared by laminating a thermoplastic resin film on a surface-treated steel sheet in advance.
Since the surface-treated steel sheet is laminated on its two sides with the thermoplastic resin film, therefore, this thermoplastic resin film plays the role of a lubricant when the bottomed cylindrical can is to be formed by the drawing working or the thinning working, when the can is drawn on its bottom side or when the neck portion is threaded. When the surface-treated steel sheet is extended or bent, the laminating thermoplastic resin film layer is accordingly extended or bent to reduce the consumption of the lubricant and to make the surface-treated steel sheet hard to be damaged with the working tools.
Since the laminating state with the thermoplastic resin can be kept even after the bottle-shaped can was formed, moreover, no new protecting coating is required later.
In the method of the invention, moreover, the bottomed cylindrical can is so thinned that the thickness of the side wall in the vicinity of its bottom is less than the thickness of the metallic sheet before formed, but is 60% or more of the thickness of said metallic sheet.
As a result, the thickness of the side wall in the vicinity of the bottom of the bottomed cylindrical can or the portion to be drawn at the multiple stages is less than the thickness of the metallic sheet before the can is formed, but is 60% or more of the thickness of said metallic sheet. As a result, the corner portion is hard to be wrinkled, when it is preformed into the curved shoulder face, so that the joint portion from the trunk portion to the shoulder portion can be smoothly curved.